This project assesses the modulatory role of endogenous steroids on GABA- A receptors and GABA-A-modulated biological function based on receptor, cellular and tissue studies. The de novo synthesis of steroids in the CNS, such as dehydroepiandrosterone sulfate (DHEAS), suggests a functional role which remains to be identified. Using brain membranes, DHEAS receptor binding was characterized biochemically and pharmacologically. DHEAS bound to two sites, with the high affinity site associated with protein and the GABA-A receptor complex. Electrophysiological studies showed DHEAS dose-dependently inhibited GABA-A-induced chloride currents in cultured neurons. The IC50 of 10 mu-M corresponded with the affinity of the high affinity binding site of 3[H]-DHEAS. The results suggest DHEAS functions as a noncompetitive negative modulator of the GABA-A receptor in the CNS, enhancing neuronal excitability. The biological activity of the steroids, tetrahydroprogesterone (THP) and pregnenolone sulfate (PS), were tested on spontaneous contractions recorded from insulated tissue strips of rabbit uterus, in which GABA is a regulator of uterine motility, and were compared to established GABA ligands. The GABA-A agonist steroid THP inhibited while the GABA antagonist steroid PS increased contractions. THP antagonized the stimulatory effect of PS, but progesterone inhibition of PS was delayed suggesting that the inhibition was mediated via the progesterone metabolite, THP. This novel mechanism may play a role in uterine function during pregnancy and parturition.